Method for ultrafiltration of blood

ABSTRACT

A method for filtering blood including: withdrawing blood from an adult patient; performing ultrafiltration by filtering the withdrawn blood in a filter having an active filter membrane surface of no greater than 0.2 meters squared (m 2 ) to remove ultrafiltrate from the blood, wherein the filter membrane blocks the passage of blood molecules having a molecular weight of at least 60,000 Daltons, wherein an amount of the removed ultrafiltrate is an effective therapeutic amount for treating a fluid overload condition of the patient; infusing the ultrafiltrated blood into the adult patient, and pumping the ultrafiltrate from the filter a rate no greater than one liter per hour.

RELATED APPLICATION

This application is a divisional of application Ser. No. 13/098,853filed May 2, 2011, which is a divisional of Ser. No. 11/960,145 (nowU.S. Pat. No. 7,935,071), filed Dec. 19, 2007, which is a continuationof application Ser. No. 10/642,638 (now U.S. Pat. No. 7,311,689), filedAug. 19, 2003 which is a divisional of application Ser. No. 09/618,759,filed Jul. 18, 2000 (now U.S. Pat. No. 6,890,315) and claims priority tothe Provisional Application Ser. No. 60/206,232, filed May 23, 2000, theentirety of all of these applications are incorporated by reference.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for treatment ofcongestive heart failure (CHF). In particular, the invention relates tothe removal of excessive fluids, such as water, in patients sufferingfrom CHF.

BACKGROUND OF THE INVENTION

Congestive Heart Failure (CHF) is the only form of heart disease stillincreasing in frequency. According to the American Heart Association,CHF is the “Disease of the Next Millennium”. The number of patients withCHF is expected to grow even more significantly as an increasing numberof the “Baby Boomers” reach 50 years of age.

CHF is a condition that occurs when the heart becomes damaged andreduces blood flow to the organs of the body. If blood flow decreasessufficiently, kidney function becomes impaired and results in fluidretention, abnormal hormone secretions and increased constriction ofblood vessels. These results increase the workload of the heart andfurther decrease the heart's pumping ability and, that in turn, causesfurther reductions in blood flow to the kidney. It is believed that theprogressively-decreasing perfusion of the kidney is the principalnon-cardiac cause perpetuating the downward spiral of the “Vicious Cycleof CHF”. Moreover, the fluid overload and associated clinical symptomsresulting from these physiologic changes are the predominant cause forexcessive hospital admissions, terrible quality of life and overwhelmingcosts to the health care system due to CHF.

While many different diseases may initially damage the heart, oncepresent, CHF is split into two types: Chronic CHF and Acute (orDecompensated-Chronic) CHF. Chronic Congestive Heart Failure is a longerterm, slowly progressive, degenerative disease. Over years, chroniccongestive heart failure leads to cardiac insufficiency. Chronic CHF isclinically categorized by the patient's ability to exercise or performnormal activities of daily living (such as defined by the New York HeartAssociation Functional Class). Chronic CHF patients are usually managedon an outpatient basis, typically with drugs.

Chronic CHF patients may experience an abrupt, severe deterioration inheart function, termed Acute Congestive Heart Failure, resulting in theinability of the heart to maintain sufficient blood flow and pressure tokeep vital organs of the body alive. These acute CHF deteriorations canoccur when extra stress (such as an infection or excessive fluidoverload) significantly increases the workload on the heart in a stablechronic CHF patient. In contrast to the stepwise downward progression ofchronic CHF, a patient suffering acute CHF may deteriorate from even theearliest stages of CHF to severe hemodynamic collapse. In addition,Acute CHF can occur within hours or days following an Acute MyocardialInfarction (AMI), which is a sudden, irreversible injury to the heartmuscle, commonly referred to as a heart attack.

A. Treatment Strategies for CHF

1. The Treatment of Chronic CHF

There are currently two broad categories for the treatment of ChronicCHF: (1) drug therapy and (2) surgical therapy. All treatments share thecommon goal of the alleviation of CHF symptoms, the improvement of heartfunction, and the disruption of the neurohormonal secretions of a kidneyto decrease stress and prevent possible failure.

A cornerstone of the drug therapy of Chronic CHF includes the use ofangiotensin converting enzyme (ACE) inhibitors, positive inotropicagents, diuretics, digitalis, and, more recently, beta-blockers with theamount of each drug used dependent on the stage of heart failure.

Positive Inotropic Agents

Directly combating the inability of the heart to propel blood forwardmight seem to be the single most intuitive means for treating heartfailure. A class of drugs known as inotropes increases the strength ofcontraction of the remaining viable heart muscle, allowing the heart toexpel more blood with each beat. While all types of inotropes (e.g.,dobutamine, dopamine, milrinone) are effective in the short-term, theylack long-term value in the treatment of heart failure because they,like the vasodilators, tend to cause additional neurohormonal activation(as evidenced by hormonal kidney secretions) and perpetuation of thedownward spiral.

Diuretics

Diuretics decrease the sodium and water retention in a patient bypreventing reabsorption of these substances at specific sites in therenal tubules of the kidney. Diuretics, such as Lasix and Bumex, areeffective at reducing symptoms of heart failure due to fluid overload,especially in the lungs and extremities. In the long-term, diuretictherapy fails because it further activates the renin-angiotensin system(e.g., the hormones secreted by the kidney) and eventually overwhelmsthe ability of diuretics to control salt and water retention.

Vasodilators

The next logical step in the treatment of heart failure is to limitvasoconstriction and reduce its adverse effect on the heart.Unfortunately, vasodilatory agents, like diuretics, fail after a periodof time, as they decrease kidney perfusion pressure and activate therenin-angiotensin system.

ACE Inhibitors and β-Blocking Drugs

In the past two decades, the development of angiotensin convertingenzyme (ACE) inhibitors and β-blockers has signaled perhaps the mostsignificant development of this century in the pharmacological treatmentof heart failure. Both are aimed at the neurohormonal axis of thisdisease and both act by disruption of the feedback loops thatcharacterize heart failure. β-blockers and ACE inhibitors are the firstclasses of drugs to be associated with a survival benefit for patientsin heart failure. However, despite these significant advances in medicaltherapy, their effectiveness is limited, especially in the later stagesof CHF. Patients become resistive to the increased dose and potency ofdrugs until further increase becomes too dangerous.

Surgical Therapies

There are three potential surgical treatments for patients in heartfailure: (1) revascularization, (2) implantation of a heart assistdevice, and (3) heart transplantation. Revascularization is therestoration of blood flow to the heart itself, either angiographically(PTCA) or surgically (CABG). Revascularization is performed in patientsin whom it is believed that a poor blood supply to the heart itself isthe major cause of the observed heart dysfunction. A second surgicalmodality is the placement of an implantable pump that replaces thefailed ventricle. This type of device is known as the Left VentricularAssist Device (LVAD). The third and ultimate surgical modality forpatients suffering from heart failure is transplantation. While this canbe an effective means to cure heart failure, transplantation is repletewith significant medical issues. In addition, for a majority of patientssuffering from CHF, transplantation is not available because they failto meet current criteria for heart transplant recipients, socioeconomicissues and, most commonly, lack of donor organs. Despite their benefit,surgical therapies are used in less than 1% of all heart failurepatients due to their cost, invasiveness and a lack of donors' hearts.

2. Treatment of Acute CHF

Pharmacologic Therapies

In contrast to the treatment of Chronic CHF, the abruptness and severityof the decrease in blood flow and pressure put the vital organs, e.g.,the kidneys, at immediate risk of severe damage. Interestingly, thephysiologic effects of some therapies used to treat Acute CHF closelyparallel those of the “Vicious Cycle” of CHF in that they maytransiently or permanently damage some organs to preserve the heart andthe brain.

The first line of therapy for Acute CHF is the use of intravenous (IV)inotropic (“squeeze” enhancing) agents in concert with intensivediuretic therapy. The purpose of this therapy is to substantiallyincrease the output of the heart, increasing kidney blood flow, andthereby increasing urine output. It may take hours to days for thistherapy to restore hemodynamic stability and fluid removal. In addition,therapy with IV inotropic agents has side effects. Most inotropic drugscan cause vasodilation and hypotension (low blood pressure) that maylead to Acute Renal Failure. Cardiac arrest from inotropic-inducedarrhythmias (irregular heartbeats) also can occur.

The second-line therapy is the use of vasopressor (vasoconstricting)agents. While they increase blood pressure, in the higher doses used inAcute CHF, these agents cause severe vasocontriction, and can lead tokidney and liver failure. In concert with a large mandatory fluid intakefrom multiple IV medications, progressively increasing diureticunresponsiveness and concurrent hemodynamic instability, reduced renalperfusion leads to the refractory (drug resistive) fluid overload stateseen in Acute CHF. This excess fluid decreases ventricular function,oxygenation and other organ function, and impairs the ability to givesuch additional therapies as increased IV pharmacologic therapy (such asvasopressors) or parenteral nutrition.

3. Mechanical Fluid Removal Therapies

Once pharmacological therapy is exhausted, Continuous Renal ReplacementTherapy (CRRT) has been used to treat patients suffering from excessfluid overload. CRRT has been performed previously using standardmethods of hemodialysis and continuous arterio-venous hemofiltration(CAVH). More recently, continuous veno-venous hemofiltration (CVVH) hasbeen used to reduce the complications associated with such issues ashemodynamic instability and need for arterial access.

In cases where drug therapy is no longer sufficient to support thepatient, effective, intra-aortic balloon pumps (IABPs) are commonlyused. IABPs provide limited support of blood flow and pressure in CHF.Other devices, including ventricular assist devices are invasive andcostly, but effective at increasing blood flow. Implantation of thesedevices generally requires the patient to undergo heart transplantation.

4. Failure of the Current Treatments for CHF

Current treatments for CHF share the common goal of the alleviation ofsymptoms, and the improvement of heart and kidney function. Thecornerstone of the medical therapy of chronic CHF includes the use ofangiotensin converting enzyme (ACE) inhibitors, positive inotropicagents, diuretics, digitalis, and more recently, beta-blockers with theamount of each drug used dependent on the stage of heart failure. Whiledrug therapy is effective in the early stages of CHF, there is no trulyeffective drug treatment for the later stages of CHF. Acute CHF isgenerally treated with intravenous inotropic (“squeeze” enhancing) andvasopressor (blood pressure raising) agents in concert with intensivediuretic therapy. If this therapy fails, the patient can quickly developsevere fluid overload and suffer rapidly-worsening heart and kidneyfunction. Intra-aortic balloon pumps (IABPs) are commonly used but ofminimal benefit in CHF. Hemodialysis and hemofiltration have been shownto be effective in removing extra fluid, reducing symptoms and improvingheart function, but its use is limited to the Intensive Care Units (ICU)patient population.

Surgical solutions exist, but are only used for the treatment of veryend-stage heart failure. These therapies (such as LVADs) are veryeffective at increasing blood flow. However, they are invasive, costlyand require the patient to undergo heart transplantation. Even with thewide variety of existing therapies, over 2,300,000 CHF patients becomehospitalized each year at a cost of over $10 billion dollars to thehealth care system. New CHF therapies are needed.

B. A Large Unmet Clinical Need in Patients with CHF for Enhanced FluidRemoval

If excessive fluid is not promptly removed with medication, CHF patientsare often intubated and placed on a ventilator. If the initial diuretictherapy has little affect, more aggressive treatment with increasinglypotent diuretics is needed. In addition, inotropic agents such asdobutamine are administered to increase the pumping function of theheart and rise the blood pressure. Higher blood pressure is expected toassist in the perfusion of the kidneys and make diuretics work. In morerecent years vasodilator therapy became a part of the standard therapyfor a severely volume-overloaded, decompensated CHF patient. All theabove-mentioned therapies as a rule require admission to the ICU.Potentially dangerous side affects of drugs, needed for advancedmonitoring and intubation, are the main reasons for a typical ICUadmission.

While there are many potential factors that cause a patient to behospitalized, the primary causes of admission in CHF patients aresymptoms of severe shortness of breath from fluid overload. Standarddrug therapy is unable to remove excess fluid rapidly enough to preventhospitalization before any increased standard medical therapy has timeto work. There is a clear and unmet clinical need for a CHF treatmentthat allows physicians to rapidly, controllably and safely remove aclinically significant amount of fluid from a CHF patient. Such atreatment would reduce the need for excessive hospital admissions.

Symptoms of fluid overload are excessive fluid retained in the abdomen,legs and lungs. Of these, fluid in the lungs is the most important.Patients have difficulty breathing. Edema in the lungs leads to poorblood oxygenation. Poor oxygenation leads to acidosis and deleteriousneurological and hormonal phenomena that increases vasoconstriction andload on the heart. In addition, vasoconstriction leads to reduced bloodflow to the kidneys and diminishes the effectiveness of the mainpharmacological means of fluid removal—diuretic treatment. Thisphenomenon is known as the “vicious cycle” of CHF heart failure.

As previously mentioned, hemodialysis and hemofiltration can be used toremove excess fluid from a patient, especially in patients whose kidneysare not working. The term “Renal Replacement Therapy” generally refersto any forms of dialysis, solute and fluid balancing therapy. Thesetreatments circumvent the kidney and replace kidney functions. Thesetreatments are not generally applicable to CHF patients havingfunctional kidneys, but which lack sufficient blood flow to properlyperform their kidney functions, especially the removal of excess fluids,e.g., water, from the body.

1. Principles and Concept of Existing Methods of Renal ReplacementTherapy

Renal replacement therapy performs two primary functions:ultrafiltration (removal of water from blood plasma), and soluteclearance (removal of different molecular weight substances from bloodplasma). The filter called “dialyzer” can be set up to perform either orboth of these functions simultaneously, with or without fluidreplacement, accounting for the various modes of renal replacementtherapy. “Clearance” is the term used to describe the removal ofsubstances, both normal and waste product, from the blood.

Ultrafiltration is the convective transfer of fluid out of the plasmacompartment through pores in the membrane. The pores filter electrolytesand small and middle sized molecules (up to 20,000 to 30,000 daltons)from the blood plasma. The ultrafiltrate output from the filtrationpores is similar to plasma, but without the plasma proteins or cellularcomponents. Importantly, since the concentration of small solutes is thesame in the ultrafiltrate as in the plasma, no clearance is obtained,but fluid volume is removed.

Dialysis is the diffusive transfer of small solutes out of a bloodplasma compartment by diffusion across the membrane itself. It occurs asa result of a concentration gradient, with diffusion occurring from thecompartment with higher concentration (typically the blood compartment)to the compartment with lower concentration (typically the dialysatecompartment). Since the concentration of solutes in the plasmadecreases, clearance is obtained, but fluid may not be removed. However,ultrafiltration can be combined with dialysis.

Hemofiltration is the combination of ultrafiltration, and fluidreplacement typically in much larger volumes than needed for fluidcontrol. The replacement fluid contains electrolytes, but not othersmall molecules. Since the net effect of replacing fluid without smallsolutes and ultrafiltration of fluid with small solutes results in netremoval of small solutes, clearance is obtained.

While effective at removing excess fluid, substantial clinical dataexists showing that ultrafiltration provides significant other benefitsto patients with CHF. These benefits include the promotion a variety ofcompensatory neurohumoral mechanisms, such as activation of therenin-angiotensin-aldosterone system and stimulation of the sympatheticnervous system, resulting in both sodium accumulation and increasedperipheral vascular resistance. Commonly, fluid removal with diureticsfurther enhances the neurohumoral stimulation and may even aggravateheart failure in some patients. Ultrafiltration interrupts this viciouscycle and represents an alternative approach to the treatment ofrefractory heart failure. Further beneficial effects of ultrafiltrationinclude a subsequent increase in urine output, and an increasedresponsiveness to standard oral diuretic therapy.

For example, one study randomized congestive heart failure (NYHA ClassII to III) to treatment with ultrafiltration (1.3 to 2.6 L over 3 to 5hours) or with furosemide (potent diuretic). Both treatments producedsimilar hemodynamic and fluid losses. However, three months afterintravenous furosemide treatment, hemodynamics and fluid volume hadworsened back to baseline values, yet they were still significantlyimproved in the ultrafiltration group. The data suggest that fluidremoval by ultrafiltration shifts the abnormal set point for fluidbalance to a more physiologic level, an effect not accomplished byfurosemide, despite comparable amounts of volume removal. Several otherclinical studies showed similar beneficial results. Thus,ultrafiltration appears to be a beneficial in patients with CHF, eventhose still responsive to standard medical therapy.

2. Limitations of Existing Methods of Ultrafiltration to Treat CHF

Ultrafiltration has not been used widely in the treatment of patientswith CHF, despite its apparent clinical benefits. There are severalissues limiting the use of currently available ultrafiltration devices:

i. Prior ultrafiltration devices require central venous access (e.g.,via surgery) with its attendant risk of infection, bleeding, collapsedlung and death.

ii. Prior ultrafiltration treatments require interaction and use by anephrologist who is not the patient's primary physician, and who may bereluctant to expend their device and personnel resources on thesepatients.

iii. Prior ultrafiltration devices draw large blood volumes of blood outof the body and, thus, require central venous access. Moreover, thetemporary large blood loss may lead to hypotension (low blood pressure)and the potential for large losses of blood.

iv. Prior ultrafiltration devices are generally designed to be only usedin the ICU or dialysis unit environment.

v. Prior ultrafiltration devices require high blood volume flows toprevent clotting in the blood circuit and filter apparatus.

vi. Most patients are required to be anticoagulated leading to anincreased risk of bleeding.

Continuous Veno Venous Hemofiltration (CVVH) allows removal of bloodfluid and modification of the volume and composition of extracellularfluid to occur evenly over time. A filter that is highly permeable towater and small solutes but impermeable to plasma proteins anderythrocytes, is placed in the extracorporeal circuit. As the bloodperfuses the hemofilter an ultrafiltrate is removed in a manner similarto glomerular filtration in the kidney.

Modern CVVH machines over time can provide almost complete renalreplacement therapy (act as an artificial kidney) in an anuric patient.The technique is typically used in the ICU setting on a patient that haspermanently or temporarily lost natural renal function as an alternativeto intermittent dialysis. Secondary to the artificial kidney function,CVVH offers precision and stability that allows electrolytes or anyappropriately sized element of circulation to be removed or addedindependently of changes in the volume of body water. In turn, ifdesired, the volume of water can be adjusted in a controlled fashion.Although valuable and powerful clinical tool, the versatility of CVVHlimits its use in clinical practice and acceptance. The limitations ofCVVH include:

i. Electrolyte removal and replacement is a high risk therapy. It isparticularly risky in cardiac patients, since excess or depletion ofelectrolytes can cause arrhythmias. For this reason primarily, CVVH isprescribed and administered by a nephrologist.

ii. Large amounts of blood, in the range of 100-400 mL/min or as much as10% of the total cardiac output for an adult patient, are passed throughthe filter. This necessitates the so-called “central” vascular access.Relatively-large and long catheters are threaded from a peripheral veinin an arm or a leg of the patient until they reach a large vascularvolume in the center of the body where the sufficient blood flow ispresent. These cavities containing large volumes of blood can be thevena cava or the right atrium of the heart. To establish the centralaccess a vascular surgeon or another similar specialist is required.Patients with central access catheters require additional monitoring.The central access is associated with serious complications.

iii. Tens of liters of fluid are continuously removed and replaced in apatient over the course of one day. If the desired balance is disrupted,patient can rapidly gain or lose fluid. The part of the machineresponsible for fluid balance is called an ultrafiltration controller(UC). The UC of a modern CVVH machine has evolved into an extremelysophisticated apparatus capable of measuring the rate at which fluid isadded or removed with the accuracy of less than 0.5%. This accuracycomes at a price of technical complexity and cost.

As a result, CVVH has in the past only been used in the ICU of ahospital where resources, training and adequate nursing monitoring areavailable. In addition, controls of CVVH machines are difficult tounderstand and require extensive training Although the latest and mostsophisticated apparatus “Prisma” from Gambro takes advantage ofinteractive computer screens to simplify the task of setting up thedevice and controlling its use, it still requires many parameters to beconfigured before the device can be used.

Mechanical fluid removal such as SCUF (Slow Continuous Ultrafiltration)or CVVH is not used in these patients until it becomes obvious that thenatural kidney function is insufficient. This typically is a result ofan Acute Renal Failure (ARF) secondary to hypotension and hypoperfusionof the kidneys. The CVVH is prescribed by a nephrologist.

Abundant scientific and clinical evidence exists that aggressive earlyfluid removal with a machine would benefit CHF patients. It can reducesymptoms of fluid overload, prevent intubation, reduce load on the heartand reduce neuro-hormonal stimuli that drive vasoconstriction. To betruly affective the treatment shall precede the onset of ARF and shallbe placed in the hands of a cardiologist who is the primary physicianresponsible for a CHF patient.

Current clinical use of CVVH in heart failure can be described as “toolittle too late”. As a result, CVVH is used for many days not as muchfor fluid removal but as an acute renal replacement therapy (similar todialysis) in patients with lost renal function.

With the increasing prevalence of decompensated CHF and the increasedcost of hospital admission and even more so of ICU treatment, a strongneed has emerged for a new technology that will allow fluid removal inthe non critical care setting. This need is for a device and techniquethat is simple and safe so that it could be used in the outpatientsetting, doctors offices, Emergency Rooms (ER) and general hospitalfloors.

SUMMARY OF THE INVENTION

To better address this important therapeutic modality and eliminatethese significant limitations, the inventors have developed a novelmethod and device that safely performs ultrafiltration using only theblood drawn from a peripheral vein.

One advantage of an embodiment of the invention is that it does notrequire surgery or ICU, and can be administered to patients in emergencyrooms, clinics and other such facilities. The majority of the total ofthe 2.5 million CHF patients admitted per year in the United States donot require an ICU. The admission of many of these cases to the hospitalcould be adverted altogether if there existed a safe, simple means toremove excess fluid from these patient to relieve the CHF symptoms offluid overload.

An embodiment of the invention includes a device that is suited toextract excess fluid from a CHF patient at a clinically relevant ratethat will overcome the deficiencies of existing machines for renalreplacement therapy and particularly could be used outside of an ICU.The embodiment of the invention provides a simple excess fluidextraction (SAFE) system for use with CHF patients.

The embodiment of the invention avoids the need for a central access tovenous blood. Central access catheters such as 10 or 14FR catheters fromMedcomp PA can are primarily used in the ICU or special dialysissetting. Thus, a device requiring central access to venous blood wasgenerally not used to treat CHF patients, other than the most sick CHFpatients.

An embodiment of the invention provides an acceptable level ofinvasiveness for a fluid removal treatment in the desired environmentand patient population via a peripheral vein preferably in an arm of apatient. Such access is commonly established by a nurse to draw blood orto infuse drugs. An embodiment of the invention does not require an ICUor a special dialysis setting to be administered to a patient. If anapparatus for slow continuous ultrafiltration was available that woulddraw and re-infuse blood into the body using the access site similar toa common IV therapy, such device would have a widespread clinical use.

Extracorporeal blood treatment are known in the prior art where blood iscontinuously withdrawn from, processed and returned into the same ordifferent vein in the patient's arm. For example, such methodology iscommonly used in blood apheresis treatment. Examples of apparatus forplasma apheresis are Spectra or Trima from Gambro. The major limitationof peripheral access is the relatively modest amount of blood that canbe withdrawn per unit of time. It is accepted that in almost allpatients the blood flow of 40-60 mL/min can be established. In somecases blood flow of up to 100 mL/min can be achieved. Blood flowavailable from a peripheral vein is therefore a great deal lower thanthe blood flow of 100-400 mL/min that is required to operate renalreplacement therapy machines such as Prisma or BP11 in an adult patient.

This apparently insufficient blood flow from a peripheral vein wasperceived by the engineering and medical community as a prohibitingfactor. In spite of an apparent clinical need, the peripheral veinultrafiltration was never developed or even investigated. Rather thantake the blood flow requirement on the “face value”, applicants analyzedthe medical and engineering considerations behind the requirement forblood flow. They developed a clinically-useful method and apparatus forfluid removal that can operate at blood flows of less than 100 mL/minand preferably of 40 to 60 mL/min.

Existing renal replacement therapy machines and specifically ones usedin acute setting to perform SCUF and CVVH therapy were all designed toprimarily perform hemofiltration and hemodialysis, not fluid removal.Blood is composed of cellular components suspended in the fluidcomponent called plasma. Water is the primary constituent of plasma inwhich physiological solutes such as sodium and potassium are dissolved.In plasma the larger molecules, proteins and blood cells, are suspended.Ultrafiltration and hemofiltration operate by convection. Inultrafiltration, a solute molecule is swept through a membrane by amoving stream of ultrafiltrate. Proteins and blood cells are retained bythe membrane. In patients with renal failure, renal replacement therapy,such as hemofiltration or dialysis, removes undesired solute. In renalreplacement therapy, vital elements such as electrolytes are removedfrom the blood and need to be replaced.

During hemofiltration solute removal is entirely dependent on convectivetransport. Hemofiltration is relatively inefficient for solute removal,as compared to dialysis. Hemodialysis allows the removal of water andsolutes by diffusion across a membrane in the direction of theconcentration gradient. Diffusion transfers solute molecules across themembrane in the direction of the lower solute concentration at the rateinversely proportional to the molecular weight.

Hemodialysis requires a large membrane surface to enable effectivesolute clearance by diffusion. Hemofiltration requires large amount ofultrafiltrate to be transferred across the membrane to remove arelatively small amount of solute. This is a consequence of convectionbeing an inefficient method of solute transport. Large amounts of fluidsuch as 1 to 4 liters per hour (L/hour) are continuously being removedduring CVVH. The resulting loss of water and electrolytes areimmediately dangerous to the patient. To maintain fluid and electrolytebalance, equally large or slightly lower amount of replacement fluid isinfused into the patient. Replacement fluid is thus added into theextracorporeal blood circuit before or after the filter.

There is a straightforward dependency between the maximum amount ofultrafiltrate removed from the blood and the flow (volume per unit time)of blood that must pass through the filter. Blood condenses in thefilter as the water is removed. The water is removed as it is sievedfrom the blood in the filter. In practice only approximately 20% to 30%of the ultrafiltrate volume can be removed from blood safely as water.If more is removed, the blood becomes too dense with red blood cells andprotein and will flow sluggishly.

Also, filter membranes are designed to pass water and small solutes asultrafiltrate, but to retain red blood cells and proteins. Thisinherently limits the amount of fluid that the filter can remove perunit of surface area of the membrane. The permeability of a filter toultrafiltrate per unit of driving Trans Membrane Pressure (TMP) is aconstant called the ultrafiltration coefficient (KUF) of a filter. TheKUF of blood filters has increased dramatically in recent years. Inmodern CVVH machines filters with KUF as high as 50 mL/hour/mmHg areused. For example, the popular Fresenius F-series of filters have KUF of20, 30 and 40. The higher KUF of filters was achieved by using morepermeable membranes. Permeability of fibers is measured inmL/hour/m2/mmHg. The permeability of fiber used in Fresenius filters is33 mL/hour/m2/mmHg. Higher permeability membranes allow efficientultrafiltration with smaller membrane surface areas. This method ofincreasing efficiency of ultrafiltration has inherent limitations. KUFof a membrane substantially higher than 50 mL/hour/m2/mmHg cannot beachieved since the filter will eventually become permeable to proteins.

In general, an increase in the flow of ultrafiltrate requires a largerfilter membrane surface. The rate of ultrafiltration can be somewhatimproved by increasing the TMP, but the effectiveness of this method isalso limited. When TMP increases over a value of 200-300 mmHg, theultrafiltration rate stops increasing and will actually start droppingif TMP is increased much further. This is explained by the fouling ofthe membrane. Proteins are pushed into the pores of the membrane byhigher TMP and the actual KUF of the membrane becomes lower than thetheoretical KUF. Eventually the membrane is clogged and theultrafiltration stops. Also high TMP leads to hemolysis and distructionof red blood cells.

These limitations are well known and accepted in the industry. As aresult, filters used for CVVH in adults have the membrane surface areaof 0.7-2 m2. The required minimal surface area drives several designconstraints of the filter. A substantial number of hollow membranefibers (typically >3,000) are used in parallel to create the desiredsurface area in a bundle of practical length. This determines the areaof the contact of blood with an artificial material and the time of theexposure of blood to that material for a given blood flow. Prolongedexposure to plastics is known to trigger the process of coagulation ofblood. Unless undesirable high doses of anticoagulants such as heparinare used, blood flowing slowly through a filter with a substantialmembrane surface area will inevitably clot. In fact, filter clotting isthe number one problem of both intermittent dialysis and continuoushemofiltration. To prevent blood from clotting in the filter substantialblood flows in access of 100 mL/min are maintained through theextracorporeal circuit. As a result, although CVVH machines such asGambro Prisma can, according to the specification, pump blood at flowsas low as 10 mL/min, these flows are not practical and are never used inadults.

The above-mentioned facts frame an unsolvable problem for a designer ofa CVVH machine that would function reliably for many hours and drawblood from a peripheral vein access. The blood flow available is notsufficient to provide the clinically useful rate of solute removal usingany existing filter material or design. In addition, the highextracorporeal flow of blood is likely to require close monitoring ofthe patient even in the presence of a sophisticated ultrafiltrationcontroller. The existing ultrafiltration controllers are based on eitherintegration of flow or measuring weight of removed and added fluid. Bothmeasurement methodologies are dependent on calibration and are sensitiveto drift, noise and artifacts.

To overcome some of the problems with CVVH, applicants determined thatthe invention need not provide total renal replacement therapy. Patientswith fluid overload resulting from decompensated heart failure do notsuffer from acute or chronic renal failure. Their needs are differentfrom the ICU patients that the CVVH is used on. The intrinsic kidneyfunction of these patients would be sufficient to remove fluid andsolute if the vasoconstriction of the renal artery was not limiting theblood flow to the kidney. In those patients that respond well todiuretics, kidneys usually start functioning quite adequately after theinitial 2-5 liters of excessive fluid is removed.

The total renal function can in general be divided into water and soluteremoval. An embodiment of the present invention provides a water removalfunction, but does not provide solute removal. An apparatus of anembodiment of the present invention may not be capable of removingclinically significant amount of solute. The lack of solute removal inan embodiment of the present invention is beneficial in that it does notsignificantly remove electrolytes from the blood. In heart failurepatients, loss of electrolytes can lead to dangerous arrhythmias.Moreover, even with very low daily urine output the kidney is capable ofthe solute clearance that is sufficient to sustain metabolicequilibrium. Kidney can concentrate solute in as little as 0.6 litersper day of urine. However, this amount of urine output is not sufficientto maintain the fluid balance. Accordingly, the kidneys of a CHF patientmay be sufficient for solute removal needs, and require the assistanceof the present invention for water (fluid) removal.

SUMMARY OF THE DRAWINGS

A preferred embodiment and best mode of the invention is illustrated inthe attached drawings that are described as follows:

FIG. 1 illustrates the treatment of a fluid overloaded patient with anembodiment of the present invention.

FIG. 2 illustrates the operation and fluid path of the preferredembodiment of the present invention.

FIG. 3 is a graphical illustration tracing how the desired averageultrafiltration rate of 250 mL/hour is achieved with a 50% duty cyclecontrol.

FIG. 4 illustrates another embodiment of the invention when more preciseremoval of fluid is desired.

FIG. 5 is a graph comparing the effect of the reduction of the filtermembrane area on the residence time of blood by comparing transit timeof blood in contact with plastic tubing, a standard F40 filter, and thefilter described herein.

FIG. 6 is a graphical representation illustrating that the desiredamount of ultrafiltrate can be removed with the filter described in thepreferred embodiment.

FIG. 7 is an illustration of another embodiment of the invention with anapparatus designed for removing blood in a batch type process using asingle needle and blood storage bag.

DETAILED DESCRIPTION OF THE INVENTION

For the proposed clinical use, the capability of the disclosedembodiment of the invention is to remove water drove the design. This isfortunate since the convection technique proposed here for removal ofwater is inefficient (in terms of the solute material removed per unitof the surface area of the membrane) in removing solute from the bloodplasma. The functional kidneys of the patient are relied on to removesolutes from the plasma. The kidneys are, thus, relived of having toperform substantially all fluid removal from the blood.

FIG. 1 illustrates the treatment of a fluid overloaded patient with theembodiment of the invention 100. Patient 101 can undergo treatment whilein bed or sitting in a chair. Patient can be conscious or asleep. Toinitiate treatment two relatively standard 18G needles 102 and 103 areintroduced into suitable peripheral veins (on the same or differentarms) for the withdrawal and return of the blood. This procedure is nodifferent from blood draw or IV therapy. Needles and attached to tubing104 and 105 and secured to skin with attachments 106 and 107. The bloodcircuit that consists of the blood filter 108, tubes, pressure sensors109, 110 and 111 and the ultrafiltrate collection bag 112. The circuitis supplied in one sterile package and is never reused. It is easy tomount on the pump 113 and can be primed and prepared ready for operationwithin minutes by one person.

During operation, the disclosed embodiment of the invention requiresminimal intervention from user. User sets the maximum rate at whichfluid is to be removed from the patient using the control panel 114.Ultrafiltrate is collected into a graduated one-liter collection bag112. When the bag is full, ultrafiltration stops until the bag isemptied. Information to assist the user in priming, setup and operationis displayed on the LCD display 115.

FIG. 2 illustrates the operation and fluid path of the preferredembodiment of the present invention. The embodiment of the presentinvention consists of a microprocessor controlled console and adisposable kit. Disposable kit is bonded (with the exception of needles)and is supplied sterile.

Blood is withdrawn from the patient through the 18 Gage or similarwithdrawal needle 201. The needle 201 is inserted into a suitableperipheral vein in the patient's arm. Blood flow is controlled by theroller pump 204. Before entering the pump blood passes throughapproximately two meters of plastic tubing 202. Tubing is made out ofmedical PVC of the kind used for IV lines and has internal diameter (ID)of 3 mm. Pump is rotated by a DC motor under microprocessor control. Thepump segment (compressed by the rollers) of the tubing has the same IDas the rest of the blood circuit. The system is designed so thatapproximately 1 mL of blood is pumped per each full rotation of thepump, e.g. pump speed of 60 RPM corresponds to 60 mL/min.

The disposable withdrawal pressure sensor 203 is a part of the bloodcircuit. Pressure sensor 203 is a flow-through type commonly used forblood pressure measurement. There are no bubble trap or separationdiaphragms in the sensor design, which reduce the accuracy. Pressuresensor is designed to measure negative (suction) pressure down to −400mmHg. All pressure measurements in the fluid extraction system arereferenced to atmospheric. The withdrawal pressure signal is used by themicroprocessor control system to maintain the blood flow from the vein.Typically, a peripheral vein can continuously supply 60-100 mL/min ofblood. This assumption is supported by the clinical experience withplasma apheresis machines.

In some cases, blood flow can be temporarily impeded by the collapse ofthe vein caused by the patient motion. In other cases the vein of thepatient may not be sufficient to supply the maximum desired flow of 60mL/min. The software in the present invention microprocessor is designedto control the withdrawal of blood to prevent or recover from thecollapse of the vein and reestablish the blood flow based on the signalfrom the withdrawal pressure sensor.

The same pressure signal from the sensor 203 is used to detect thedisconnect of the withdrawal bloodline 202 from the needle 201. Thiscondition is detected by the abrupt decrease of the withdrawal pressuregenerated by the pump. The resistance of the 18 Gage needle, which is 4cm long with an approximately 0.8 mm ID at a flow rate corresponding toa 60 mmHg, pressure drop is on the order of the 100 mmHg. The resistanceof 2 meters of blood tubing with a 3.5 mm ID at the same flow rate is onthe order of 20 mmHg. This enables automatic reliable detection of theline disconnection. The occlusion of the withdrawal bloodline isdetected in the similar fashion. The occlusion can be caused by thecollapse of the vein or by the kinked blood tube. Occlusion results in arapid decrease (more negative) of the pump suction pressure that isdetected by the microprocessor. In response to this condition, themicroprocessor stops the pump and alarms the user.

The present invention uses a double roller pump 204 to pump blood. Asthe pump 204 rotates, rollers compress the segment of PVC tubing andgenerate flow. Pump 204 is adjusted to be fully occlusive until thepressure limit is reached. The rollers are spring loaded to limit themaximum positive and negative pressure generated by pump head. Thisfeature is not normally used to limit pressure in the circuit and isonly included as a secondary safety precaution.

A direct drive stepper motor rotates the rollers, and the speed of themotor is determined by the controller microprocessor. The RPM of thepump 204 is used as a feedback signal by the controller to determine theblood flow. Normal operational blood flow in the present invention isbetween 40 and 60 mL/min. This minimum rate of blood flow is needed togenerate Trans Membrane Pressure (TMP) needed for ultrafiltration and toprevent stagnation and clotting of blood in filter 207.

Pump pressure sensor 205 is incorporated into the post-pump segment ofthe blood tubing connecting pump 204 to the blood inlet port 214 of thefilter 207. Like other pressure sensors in the present invention it is aflow through device that does not create a blood-air interface and doesnot disturb the blood flow. The pump pressure signal is used by themicroprocessor to determine TMP used to calculate the ultrafiltrationrate. It is also used to detect abnormal conditions in the circuit suchas occlusion or unacceptable clotting of the filter and disconnection ofthe blood line between the pump 204 and the filter 207.

On its way from the pump 204 to the filter 207, blood passes through theair detector 206. The air detector 206 is of ultrasonic type and candetect air in amounts exceeding approximately 50 microliters. Thedetector 206 uses technology based on the difference of the speed ofsound in liquid and in gaseous media. If an air bubble is detected, thepump 204 is stopped almost instantaneously (within few milliseconds).The bubble detector output signal is hard wired into the motor controllogic and the pump 204 is stopped independently of the microprocessorcontrol if a bubble is detected.

Air can only enter the present invention circuit from the pre-pump(negative pressure) segment of the blood circuit 202. All the rest ofthe circuit downstream of the pump 204 is always pressurized. For thisreason, the bubble detector is placed before the filter.

Blood pressure in the post pump, pre-filter segment of the circuit isdetermined by the patient's venous pressure, the resistance to flowgenerated by the return needle 210, resistance of hollow fibers in thefilter assembly 207 and the resistance of interconnecting tubing 208. Atblood flows of 40 to 60 mL/min the pump pressure is in the 300 to 500mmHg range depending on the blood flow, condition of the filter, bloodviscosity and the conditions in the patient's vein.

The filter 207 is a main component of the present invention. Inside thefilter 207 the ultrafiltration occurs. Whole blood enters the bundle ofhollow fibers from the connector on the top of the cap of the filtercanister. There are approximately 700 hollow fibers in the bundle, andeach fiber is a filter. Blood flows through a channel approximately 0.2mm in diameter in each fiber. The walls of the channel are made of aporous material. The pores are permeable to water and small solutes butimpermeable to red blood cells, proteins and other blood components thatare larger than 50,000-60,000 Daltons. Blood flow in fibers istangential to the surface of the filter membrane. The shear rateresulting from the blood velocity is high enough such that the pores inthe membrane are protected from fouling by particles, allowing thefiltrate to permeate the fiber wall. Filtrate (ultrafiltrate) leaves thefiber bundle and is collected in space between the inner wall of thecanister and outer walls of the fibers.

The geometry of the present invention filter is optimized to preventclotting and fouling of the membrane. The active area of the filtermembrane is approximately 0.1 m2. The permeability KUF of the membraneis 30 to 33 mL/hour/m2/mmHg. These parameters allow the desiredultrafiltration rate of approximately 500 mL/min at the TMP of 150 to250 mmHg that is generated by the resistance to flow. The effectivefilter length is 22.5 cm and the diameter of the filter fiber bundle is1.2 cm. This results in the shear rate of 1,200 to 1,800 sec-1 at theblood flow rate of 40 to 60 mL/min.

The TMP in the present invention is defined predominantly by theresistance of the return needle 210 and the resistance of the filterbundle inside the filter 207. The properties of the filter 207 and theneedle 210 are selected to assure the desired TMP of 150 to 250 mmHg atblood flow of 40-60 mL/min where blood has hematocrit of 35 to 50% at37° C.

The quantitative clinical goal was formulated for the apparatus beingdeveloped in terms of fluid removal. Applicants' research establishedthat for the fluid removal device to be clinically useful it shouldremove water at the rate of 100 to 500 mL/hour. Lower rates of fluidremoval are only required in hemodynamically unstable patients that aretreated in the ICU and are not the targeted patient population. Fluidremoval rates higher than 500 mL/hour (theoretically as high as 1,000mL/hour) may be practical in some patients but are expected to be toohigh risk in the majority. It is only advisable to remove water fromblood at the rate at which fluid can be recruited from tissue. Higherrates may lead to hypotension.

Blood hematocrit (volume fraction of red blood cells) in CHF patients isexpected to be in the range of 30 to 40% of the total blood volume. Itis possible to condense the filtered blood to the hematocrit range of50% to 60% and still be able to return blood through a standard needle.Therefore, extraction of approximately 20% to 30% of volume from bloodas water is possible. Assuming this extraction rate, the amount of bloodremoved from a peripheral vein is less than 2% of the total cardiacoutput. In addition, at this extraction rate, the potentialultrafiltrate flow may be as much as 1 L/hour. Alternatively, a lowerextraction rate, e.g., 0.1 liter/hour, may be selected. At the bloodflow rate of 60 mL/min applicants successfully extracted up to 12 mL/min(or 720 mL/hour) of ultrafiltrate in the laboratory using the filterdescribed here. Therefore, it is possible to consistently extract therequired 500 mL/hour of water from the blood flow withdrawn and returnedinto a peripheral vein.

Applicants established that the much higher blood flows that are used inadults by all existing renal replacement therapy machines andparticularly by machines for acute CVVH treatment of CHF patients arenecessitated by the filter designed to remove solute and morespecifically by the relatively high surface area of the filter. Thislarge surface area is needed for solute removal. If the goal oftreatment was to remove water only, high blood flow will still be neededto reduce the time of exposure of blood to the synthetic membrane and toprevent clotting.

Another important consideration that forces the designers of CVVHmachines to use high blood flow and consequentially the central venousaccess is the need to maintain substantially high wall shear rate ofblood flowing inside the filter capillaries (hollow fibers). Flow ofblood inside a fiber is laminar Shear rate at the wall can be calculatedusing the simple Equation 1:

y=32×Q/(d3×pi)  (Equation 1)

Q is blood volumetric mass flow rate and “d” is the internal diameter ofthe capillary.

The ultrafiltration rate is influenced by membrane fouling which is anequilibrium of wall shear rate and ultrafiltration rate per unit surfacearea. With the increasing surface area the wall shear rate will decreaseunless the blood flow is increased to compensate. It becomes apparentfrom literature that the wall shear rate should be 1,000 sec-1 or higherto achieve sufficient filtrate flux at high hematocrit. It is also knownfrom literature that the high shear rate in excess of 2,500 sec-1 isundesirable since it can cause hemolyisis and damage to red blood cells.At the same time, it is apparent that the surface area and size of thefilter should be minimized. Biocompatibility is inversely proportionalto the surface area exposed to blood. The likelihood of clottingincreases with residence time proportional to the filling volume. Also,cost of a smaller filter is lower.

To minimize the cost of the filter, the use of commercially-availablefibers with optimized biocompatibility and consistent filtrationproperties is desired. Suitable filter fiber is available, for example,from Minntech Inc. in Minnesota. Each fiber has internal diameter of 0.2mm. Pores in the fiber walls are optimized to retain solutes of greaterthan 50,000 Daltons. The permeability of this fiber is 33mL/hour/m2/mmHg. If a membrane with total surface area of only 0.1 m2 isconstructed from this fiber, the resulting theoretical ultrafiltrationrate will be 330 mL/hour at TMP of 100 mmHg and 660 mL/hour at TMP of200 mmHg. These numbers are consistent with the objective of the design.

To calculate the KUF of the filter, the permeability of fiber ismultiplied by the surface area of the membrane. Therefore KUFfilter=33×0.1=3.3 mL/hour/mmHg.

It is known from literature that the blood flow is not equal between thefibers in the filter bundle. Blood flow and consequentially the wallshear rate tends to be lower in the fibers closer to the periphery andhigher in the central ones. Accordingly, blood residence time is longerin peripheral fibers. It is known from the practice of dialysis that theperipheral fibers tend to clot first.

To reduce the extracorporeal blood volume and the time that bloodresides outside of the body it is desired to use blood lines that haveinternal diameter as small as practical without creating excessiveresistance to flow. For our application, an internal diameter of around3.0 mm is well suited. When blood exits the tubing and flows into thefiber bundle the diameter of the channel through which blood flowsincreases substantially. This creates turbulence and stagnant zones atthe entrance into the bundle. These factors increase the probability ofclotting.

It is therefore beneficial to design a filter that has a minimal butstill practical diameter of the fiber bundle. This is achieved byreducing the number of fibers and increasing the length of the bundle.This approach is limited by two constraints. Resistance of the bundle toflow increases in proportion to the bundle length. Also, long filterssubstantially in access of 20-25 cm are difficult to manufacture andcumbersome to use.

Applicants chose to use a maximum length of the filter that is practicalfrom the manufacturing standpoint. The resulting working length of thebundle is 22 cm (centimeters). To ensure the required surface area ofthe membrane, approximately 620 to 720 fibers of this length are need tobe bundled in parallel. Assuming the fiber density of approximately 630capillaries per cm2 the diameter of the bundle is 1.2 cm. Such filtercan be easily manufactured using existing methods and equipment. At theblood flow of 40 to 60 mL/min and blood hematocrit of approximately 40%,the resistance of this filter to blood flow is on the order of 100 to200 mmHg. This pressure level is acceptable for the design of a circuitwith a standard peristaltic pump and an 18 to 20 Gage (internal diameterof 0.8-1.0 mm) return needle.

Applicants overcame the perceived impossibility of clinical peripheralvein ultrafiltration that limited the use of mechanical fluid removal inCHF patients outside of the ICU environment. Applicants did this bydrastically reducing the filter membrane surface area compared to commondialysis or CVVH filters to maintain high shear rate and low bloodresidence time. Specifically, a filter with the membrane surface of lessthan 0.2 m2 and preferably 0.05 to 0.15 m2 can remove the desired 100 to700 mL/hour of water from the extracorporeal blood flow of less than 100mL/min or more specifically of 40 to 60 mL/min with an average bloodcell residence time outside the body of less than 2 minutes, and may beless than 1 minute. Although the filter is made of high permeabilityfiber due to the small surface area the KUF of the filter is less than 5mL/hour/mmHg or preferably 2 to 4 mL/hour/mmHg. Typical filters used inadult patients have KUF of 30 to 50 mL/hour/mmHg. The much lower KUFgives the present invention device design an advantage of inherentlysafer operation. Food and Drug Administration (FDA) classifies allfilters with KUF greater than 8 mL/hour/mmHg as “high permeabilitydialyzers”. According to current FDA safety standards these devices haveto be labeled for use only with ultrafiltration controllers that areindependent of TMP based ultrafiltrate rate calculation. A small errorof TMP measurement or a deviation of membrane permeability from thespecification can result in substantial over or under filtration. Theuse of a low KUF filter enables, if desired, the present invention toavoid using a cumbersome and expensive ultrafiltration controller thattypically involves a scale balance and an ultrafiltration pump.

A filter that is relatively long and narrow may optimize the blood flowinside the filter, maintain the desired wall shear rate and minimizemembrane fouling and filter clotting. A filter with a fiber bundle thatis approximately 20 cm long and 1.5 cm in diameter is particularly wellsuited for the application and is practical for manufacturing.

Filters for ultrafiltration of blood with small surface area of lessthan 0.2 m2 are known. Example of such filter is Miniflow™ 10 fromHospal. Miniflow has surface area of 0.042 m2 and KUF of 0.87mL/hour/mmHg. All such filters without exception are used forhemofiltration therapy in neonatal patients and infants. The clinicallyused amount of blood flow through these filters is within the range thatwe targeted or 10 to 60 mL/min. Nevertheless, this amount of flow—ifexpressed as a fraction of the cardiac output for infants—is the same asthe blood flow used in adult hemofiltration. Consequently, these infantfilters are used with the central and not peripheral venous access.

To minimize clotting and fouling of the membrane it is desired tomaintain substantially high blood flow through the filter even if thedesired ultrafiltration rate is low. In traditional machines for renalreplacement therapy it is typically achieved by reducing the TMP. Flowof ultrafiltrate is actively controlled by the roller pump in theultrafiltrate removal line between the filter and the ultrafiltratecollection bag. When the pump is slowed down ultrafiltrate flow isretarded, pressure gradient across the membrane is reduced andultrafiltration is slowed to the desired level. Alternatively, if thepump RPM is increased, the flow of ultrafiltrate is accelerated.Negative pressure can be developed by the pump to actively suck theultrafiltrate across the membrane. For reasons of safety and simplicity,it was desired to have a machine that can reduce the ultrafiltrationrate at the user command without an ultrafiltrate pump. For ourpreferred embodiment we used the duty cycle controlled ultrafiltration.A simple pinch valve is placed in the ultrafiltrate line. When the valveis closed, pressure across the membrane quickly equilibrates, and noultrafiltration occurs. When the valve is opened, ultrafiltration occursat the rate determined by the TMP and the KUF of the membrane. This ratecan be calculated by the controller. Valve is cycled approximately everyminute. The fraction of the cycle during which the valve remains openeddetermines the average rate at which fluid is removed.

Since the system embodying the present invention does not employ anultrafiltrate pump that can create sub-atmospheric pressure on theultrafiltrate side of the membrane, a simple and reliable method ofcontrolling the total amount of fluid removed in one treatment iterationis possible. The ultrafiltrate is collected into a sealed bag that isconnected by a tube to the ultrafiltrate collection chamber of thefilter casing. During the treatment the bag is gradually filled up withfluid. It is desired to have a bag that has a relatively small volumeand specifically volume of 0.5 to 1.5 liters. When the bag is full andits walls are fully distended, the pressure in the bag will start torise until it is equal to the average pressure of blood inside thefilter capillaries. Although some circulation of fluid is still possiblein and out of fibers the net loss of fluid is zero. Until a nurseempties the bag, no removal of fluid is possible.

Pressure sensors are used in the blood circuit to alarm thedisconnection and occlusion of blood lines. The pre- and post-filterpressure signals are also used to calculate TMP and ultrafiltrationrate. Two types of pressure measurement devices are typically used inmachines for renal replacement therapy.

Machines such as BP11 from Baxter use disposable air filled separationor drip chambers that are connected to permanently installed pressuresensors that are the part of the machine. This design introducespotentially hazardous air into the circuit. Air can cause embolism andaccelerated clotting. Also, this type of measurement is affected bygravity.

Machines such as Prisma from Gambro use flexible silicone diaphragms totransmit blood pressure to sensors once again mounted on the apparatusitself. This method overcomes the deficiencies of drip chambers.Separation diaphragms are subject to error when the travel of adiaphragm is restricted. This necessitated a complicated diaphragmpositioning system if the system is used for a substantial duration oftime. Also, a substantial area of a diaphragm (typically 2-3 cm indiameter) is required to ensure reliable transmission of pressure. At alow blood flow it is likely that a stagnant zone will form inside thediaphragm chamber that will eventually lead to clotting.

The present invention utilizes flow through disposable pressure sensors.This sensors are a part of the disposable blood circuit. They do notdisturb the laminar blood flow inside the blood line since the internaldiameter of the sensor element is the same as of the blood tubing (3mm). The sensing element is less than 2 mm in diameter and is embeddedflush in the wall of the sensor housing. The housing is bonded flushwith the internal wall of the blood line tube to form a continuouschannel. Although similar disposable blood pressure sensors (such asones made by Merit Medical of Utah) are used widely for invasive bloodpressure measurement this design has never been previously used in anapparatus for fluid removal.

The present invention is intended to provide safe, controlled fluidremoval in patients with fluid overload for up to 8 to 24 hours. Thesepatients all suffer from decompensated chronic CHF and are admitted oron the verge of admission to a hospital. Regardless of the exact natureof their disease theses patients present at the hospital with a numberof symptoms that manifest fluid overload and result in difficulty ofbreathing and pulmonary edema require immediate treatment. Thesepatients are typically 5 to 20 kg over their dry weight and, if treatedwith diuretics, can tolerate fluid removal rates of up to 0.5 L/hour foruntil symptoms are relieved.

The intended use of the present invention is to assist in the initialremoval of 2 to 4 liters of fluid that should result in the relief ofsymptoms and much improved responsiveness to medication. The presentinvention can be performed by a physician or nurse trained in the use ofthe device. Treatment can be performed in the setting of a monitoredhospital floor, outpatient clinic or Emergency Room. The presentinvention is prescribed by a cardiologist. The main idea behind thepresent invention is to remove excess water from the patient's bloodusing a well-accepted ultrafiltration technique at the same rate atwhich the surplus fluid can be recruited from the tissue.

The intended use of the present invention is slow continuous removal offluid by ultrafiltration of blood. Excessive removal of fluid can leadto hypotension and serious risks to health. If the fluid is removed fromvascular space too fast, it is equally dangerous and can lead tohypotension. The principle method of treatment with the presentinvention is to remove fluid at a rate that allows vascular blood volumeto be replenished with water that has accumulated in the interstitialspace as a result of the patient's condition. Patients that should betreated by the present invention are typically 10 to 20 kg over theirdry weight due to this excess water.

Potential excessive water loss or gain is a recognized hazard associatedwith RRT. Modern CRRT machines used in SCUF or CVVH mode can potentiallyremove and replace tens of liters of fluid from patient in a space ofseveral hours. As a result, even a small error in fluid balance canresult in severe risks to a patient. To prevent this from happeningaccurate ultrafiltration controllers are used that are based oncontinuous measurement of the weight of extracted and infused fluids.Ultrafiltration rate is adjusted accordingly by controlling the speed ofan ultrafiltration pump that can apply negative or positive pressure tothe ultrafiltrate side of the filter membrane.

In the case of the present invention, fluid gain is not a risk. Thepresent invention is designed for fluid removal only. To preventexcessive removal of fluid from the patient, the present inventionrelies on a number of inherently safe features and materials propertiesrather than the ultrafiltrate pump controller.

Ultrafiltrate rate (UF) is a function of TMP, Permeability of themembrane and the membrane surface. Membrane surface is a constant and inthe case of the present invention is 0.1 m2. Permeability of a filtermembrane is expressed as ml of ultrafiltrate per hour per unit of themembrane surface area, and may be 30 to 33 mL/hour/m2/mmHg. Theresulting KUF of the filter may be 3 mL/hr/mmHg. KUF of a filter candecrease during the operation owing to the fouling of the membrane butcan not increase unless the membrane is broken. Equation 2 takes intoaccount the affects of oncotic pressure on the ultrafiltration rate. TheKUF of the membrane is determined using animal blood at standardconditions such as hematocrit of 27%, temperature of 37° C. andappropriate concentration of protein and electrolytes. Although theseconditions do not perfectly reflect clinical conditions in all patientsit is a useful engineering approximation.

UF=TMP×(KUF×Area)  (Equation 2)

In the present invention, TMP is a function of blood flow and theresistance of circuit elements downstream, including the filter. TPM canbe calculated in real time by the microprocessor using equation 3 fromthe readings of pressure transducers.

TMP=(Pp−Pr)/2+Pr−Pg  (Equation 3)

Where Pp is the pump (pre filter) pressure, Pr is the return (postfilter) pressure and Pg is a pressure generated by the weight of thecolumn of ultrafiltrate. Given the unadjustable design of theultrafiltrate circuit, Pg is a constant. For the 20 cm level differencebetween the filter and the level of fluid in the bag Pg is 17 mmHg.

Substitution of the calculated TMP into Equation 2 gives a reasonableestimate of the ultrafiltration rate.

During the use of the present invention the operator sets the maximumallowed rate of ultrafiltration in mL/hour. Values between 100 and 500mL/hour are allowed. The present invention microprocessor establishesthe rate of withdrawal of blood in the range of 40 to 60 mL/min. Thisflow rate is determined by the quality of access. It is advantageous toestablish and maintain blood flow constant.

Based on the pressure sensor readings the TMP is calculated. This allowsthe calculation of ultrafiltrate rate for known KUF of the filter.

If the ultrafiltration rate is higher than desired it is reduced usingthe solenoid ultrafiltrate pinch valve 213 in FIG. 2. When the valve 213is closed the pressure inside the ultrafiltrate compartment of thefilter 6 rises rapidly until it is equal to the pressure in the bloodcompartment (fibers). When the system is in equilibrium, noultrafiltration occurs. The pinch valve 213 is cycled approximately onceper minute. The duty cycle (ratio of open to closed state) is calculatedratiometrically from the actual and desired ultrafiltration rate.

Traces on FIG. 3 illustrate how the desired average ultrafiltration rateof 250 mL/hour is achieved with a 50% duty cycle control. In FIG. 3,graph 300 shows time 301 versus ultrafiltrate flow rate 302. Times t1306, t2 307, t3 308, t4 309, t5 310 and t6 311 are evenly spaced withequal periods between them. The valve position 303 shows during whichperiods the valve 213 is open 304 and closed 305. For example, betweentime t1 306 and time t2 307, valve 213 is open, but between time t2 307and time t3 308, valve 213 is closed. For example, when the valve isopened ultrafiltrate flow is 500 mL/min, shown by lines 312. When thevalve is closed it is zero, shown by lines 313. Thus, using equalperiods of opened valve 213 and closed valve 213 will effectivelyachieve an average flow rate equal to half of the open valve flow rate.

Other factors limiting the ultrafiltration rate are the amount of bloodavailable for filtration and the hematocrit of the blood entering thefilter. Equation 4 can be used to calculate the return blood hemotocritHR from the blood flow BF, ultrafiltrate flow UF and withdrawal bloodhemotocrit HW.

UF=BF×(HR−HW)/HW  (Equation 4)

The safe pump 204 can deliver up to 60 mL/min of blood. The maximumamount of water that can be extracted from the blood is determined bythe hematocrit of the return blood. As the water is extracted,hematocrit increases. This in turn increases the viscosity of blood.Blood entering the filter typically has hemotocrit of 35 to 45%. Thecorresponding viscosity is approximately 3 to 4 mPa-s. Viscosity ofblood with the hematocrit of 60% is 6 mPa-s. At 70% hematocrit bloodbecomes too viscous to pass through the filter or the return needle.

The volume of the ultrafiltrate collection bag 215 determines totalamount of fluid removed from the patient. The bag 215 is connected tothe fluid removal port 211 of the filter 207 by the bonded PVC tube 212.The ultrafiltrate collection system is sealed. When the bag is full, thepressure in the bag 215 rises until it is equal to the average pressureof blood in the filter fibers. Ultrafiltration is stopped until the bagis emptied by the operator by opening the ultrafiltrate valve 216. Thevolume of the ultrafiltrate bag 215 is one liter. This automaticallylimits the maximum amount of water that can be removed from the patientwithout an operator's interaction. Bag 215 is transparent and has volumemarks that allow user to read the actual amount of fluid removed inpreferably 100 mL increments.

Blood exiting the filter 207 through the connector on the bottom of thefilter casing is continuously returned to the patient through the returnneedle 210. Blood flow leaving the filter is the same as the blood flowentering the filter if the ultrafiltrate clamp 213 is closed. If theclamp 213 is open ultrafiltration occurs and the blood is continuouslyfractured into the ultrafiltrate and more concentrated blood. Thehematocrit and the volume of returned blood are determined by theultrafiltration fraction, which is the volumetric fraction of theultrafiltrate relative to the volume of whole blood entering the filter.

Blood return circuit pressure sensor 209 serves several functions. Thereturn pressure is used in the TMP calculation that is in turn used tocalculate input data for the control of the ultrafiltration rate. It isalso used to detect a disconnected or occluded circuit. Excessivepressure signals the occlusion that can be caused by a kinked tube or aclotted needle. Since the resistance of the needle 210 is much higherthan the resistance of the blood line 208, disconnection of the needlefrom the tubing is easy to detect from an abrupt drop of the returnpressure.

FIG. 4 shows another embodiment of the invention. This embodiment ispreferred when more precise removal of fluid is desired. Anultrafiltrate pump 17 is used to pump and meter ultrafiltrate into thecollection bag 13. Weight scale 18 allows additional control andmonitoring of the amount of the removed fluid. Ultrafiltrate pressuresensor 15 is used to monitor the TMP and to detect clotting or foulingof the filter. Blood leak sensor 16 is used to detect the leakage of redblood cells across the filter membrane if the membrane is damaged. Theblood sensor is of photometric type and responded to the change of colorof the ultrafiltrate.

FIG. 5 illustrates the effect of the reduction of the filter membranearea on the transient time of blood in contact with plastic. Experimentswere performed using the thermodilution method. At the time mark 0 icedwater flowing through the system at 60 mL/min replaced the roomtemperature water. Three curves show the time delay before the coldwater reached the exit of the circuit (simulated return of blood to thepatient). Curve 510 corresponds to a continuous 2 meter long tube withinternal diameter of 3 mm with no filter at all. Curve 520 is the filterdescribed in this invention. Curve 530 is the standard F40 filter byFrescenious. Traces show that the time of contact of blood with plasticduring the treatment with proposed invention will be significantlyshorter than that with a conventional dialysis machine.

FIG. 6 proves that the desired amount of ultrafiltrate can be removedwith the filter described in the preferred embodiment. Curves 560, 550and 540 correspond to blood flows of 40, 50 and 60 mL/min accordingly.Animal blood with hematocrit of 27% at the temperature of 37° C. wasused in the experiment. TMP was between 100 and 250 mm Hg and shear rateof blood inside the filter was maintained in the 1000 to 2000 sec-1range. The desired amount of 5-10 mL/min of ultrafiltrate was filteredout of blood before it was returned into a vessel using a 18 Gageneedle. Filter had surface are of 0.1 m2 with membrane permeability of33 mL/hr/m2/mmHg supplied by Minntech. Filter bundle was made out of 700fibers 22 cm long as described previously in this invention.

FIG. 7 shows yet another potential embodiment of the invention. Thisapparatus is of a batch type and uses one needle 60. Blood is withdrawnfrom the vein and stored in the bag 51. During the withdrawal phaseroller pump 52 rotates counter clockwise. Approximately 50 mL of bloodcan be stored in the bag 51. Valve 59 is open and valve 58 is closed sothat the filter 54 is bypassed. Blood is then ejected from the bag 51through the filter 54 back into the patient's vein through the needle60. Valve 58 is opened and valve 59 is closed and the pump 52 rotatesclock wise. During this phase filtration occurs and the ultrafiltrate iscollected in the bag 55. Pressure sensors 53 and 56 are used to controlthe process so that vein does not collapse or over-extend and TMP is nottoo high. Air detector 57 is used to protect the patient from embolism.An anti-coagulant may be needed in view of the temporary storage ofblood in the bag 51.

The invention has been described in connection with the best mode nowknown to the applicant inventors. The invention is not to be limited tothe disclosed embodiment. Rather, the invention covers all of variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A method for filtering blood comprising:withdrawing blood from an adult patient; performing ultrafiltration byfiltering the withdrawn blood in a filter having an active filtermembrane surface of no greater than 0.2 meters squared (m²) to removeultrafiltrate from the blood, wherein the filter membrane blocks thepassage of blood molecules having a molecular weight of at least 60,000Daltons, wherein an amount of the removed ultrafiltrate is an effectivetherapeutic amount for treating a fluid overload condition of thepatient; infusing the ultrafiltrated blood into the adult patient, andpumping the ultrafiltrate from the filter a rate no greater than oneliter per hour.
 2. The method of claim 1 wherein the filter includes afilter body defining a passage having a length of at least 20centimeters (cm) and an interior diameter of no greater than 1.5 cm,wherein as the blood flows through the passage the blood flows acrossthe active filter membrane surface.
 3. The method of claim 1 wherein thefilter includes a blood passage having a volume less than two percent ofa cardiac output of the adult patient.
 4. The method of claim 1 whereinthe ultrafiltrate is removed at a rate determined by a mechanicalfiltrate pump and said method further comprising concentrating the bloodby removal of the ultrafiltrate.
 5. The method as in claim 1 wherein thefilter membrane blocks passage of blood molecules having a molecularweight of at least 50,000 Daltons.
 6. The method as in claim 1 whereinthe filter membrane blocks passage of blood molecules having a molecularweight of at least 30,000 Daltons.
 7. The method as in claim 1 whereinthe filter membrane blocks passage of blood molecules having a molecularweight of at least 20,000 Daltons.
 8. The method for filtering bloodcomprising: withdrawing blood from an adult mammalian patient;performing ultrafiltration by filtering the withdrawn blood in a filterto remove ultrafiltrate from the blood, wherein the filter includes anactive filter membrane with a surface area of no greater than 0.2 meterssquared (m²), the active filter membrane inhibits the passage of plasmaproteins from the blood, and an amount of the removed ultrafiltrate isan effective therapeutic amount for treating a fluid overload conditionof the patient; infusing the ultrafiltrated blood into the adultmammalian patient, and discharging the ultrafiltrate from the filter arate no greater than one liter per hour.
 9. The method of claim 8wherein the filter includes a filter body defining a passage having alength of at least 20 centimeters (cm) and an interior diameter of nogreater than 1.5 cm, wherein as the blood flows through the passage theblood flows across the active filter membrane surface.
 10. The method ofclaim 8 wherein the filter includes a blood passage having a volume lessthan two percent of a cardiac output of the adult patient.
 11. Themethod as in claim 8 wherein the ultrafiltrate is removed at a ratedetermined by a mechanical filtrate pump and said method furthercomprising concentrating the blood by removal of the ultrafiltrate. 12.The method as in claim 8 wherein the filter membrane blocks passage ofblood molecules having a molecular weight of at least 60,000 Daltons.13. The method as in claim 8 wherein the filter membrane blocks passageof blood molecules having a molecular weight of at least 30,000 Daltons.14. The method as in claim 8 wherein the filter membrane blocks passageof blood molecules having a molecular weight of at least 20,000 Daltons.15. A method for filtering blood comprising: withdrawing blood from anadult patient; removing ultrafiltrate from the withdrawn blood bypassing the blood across a surface of an active filter membrane having asurface area no greater than 0.2 meters squared (m²) and the activefilter membrane blocks the passage of blood molecules having a molecularweight of at least 60,000 Daltons, wherein an amount of the removedultrafiltrate is an effective therapeutic amount for treating a fluidoverload condition of the patient; infusing the ultrafiltrated bloodinto the adult patient, and discharging the ultrafiltrate with thefiltrate pump at a rate no greater than one liter of ultrafiltrate perhour.
 16. The method of claim 15 wherein the filter includes a filterbody defining a passage having a length of at least 20 centimeters (cm)and an interior diameter of no greater than 1.5 cm, wherein as the bloodflows through the passage the blood flows across the active filtermembrane surface.
 17. The method of claim 15 wherein the filter includesa blood passage having a volume less than two percent of a cardiacoutput of the adult patient.
 18. The method of claim 15 wherein theultrafiltrate is removed at a rate determined by a mechanical filtratepump and said method further comprising concentrating the blood byremoval of the ultrafiltrate.
 19. The method of claim 15 wherein thesteps of withdrawing, removing, infusing and discharging are performedsimultaneously.
 20. The method of claim 15 wherein the steps ofwithdrawing, removing, infusing and discharging are performedcontinuously.